1. Field of the Invention
The present invention relates to an ionic conductive material and an electrochemical device comprising the same. More specifically, it relates to an ionic conductive material and an electrochemical device having excellent high voltage stability.
2. Prior Art
Manganese dioxide lithium cells, which are representative of electrochemical devices, are in ever increasing demand because they have a high voltage of 3 V and a high energy density. Furthermore, recently, lithium ion secondary cells have been developed and the demand for them is increasing rapidly because they can give a high voltage of about 3.6 V.
Hitherto, LiClO.sub.4 was used as the electrolyte for this type of cell, but it lost some desirability because of stability concerns. Furthermore, the stability and reliability may be improved by dissolving or dispersing various salts in a polymer such as polyethylene oxide (PEO) to form solid electrolytes. However, this tends to reduce the ionic conductance considerably.
Besides LiClO.sub.4, the use of organic lithium salts such as LiB(C.sub.6 H.sub.3).sub.4, LiCF.sub.3 SO.sub.3, LiCF.sub.3 CO.sub.2, LiN(CF.sub.3 SO.sub.2).sub.2, and LiC(CF.sub.3 SO.sub.2).sub.3 has been proposed for organic electrolytes. (See the specifications of Japanese Kokai Publication No. 7-65843 (1995) and U.S. Pat. No. 5,260,143.)
However, the above LiB(C.sub.6 H.sub.5).sub.4 and the like have insufficient stability at high voltages and there is a problem in that, depending on the solvent used, the storage stability is poor. Furthermore, the liquid ionic conductive material (electrolytic solution), in which the above LiB(C.sub.6 H.sub.5).sub.4 is dissolved in an organic solvent, discolors on storage and a portion of the electrolyte solvent is polymerized. Then, as a result, a cell which uses this liquid ionic conductive material (electrolytic solution) has a problem in that the cell capacity deteriorates on storage.
Organic salts, such as LiCF.sub.3 SO.sub.3, LiCF.sub.3 CO.sub.2, LiN(CF.sub.3 SO.sub.2).sub.2, and LiC(CF.sub.3 SO.sub.2).sub.3 are much safer than LiClO.sub.4, but there are problems in that they show insufficient oxidation stability, their ionic conductivity is low, some materials cannot be used as the current collector of the electrode, and such salts are expensive.